This invention relates to the production of coated carrier particles for use in xerographic developers.
Several systems for development of xerographic images are known. One system which has been found to be very satisfactory is that of cascade development. In cascade development, a mixture of finely divided marking particles, referred to as toner particles, and of larger sized carrier particles is caused to flow over the surface of a xerographic member bearing an electrostatic latent image. The triboelectric relationship between the toner particles and carrier particles is such that the toner particles bear an electrostatic charge opposite in sign to that of the electrostatic latent image. The forces of attraction between the electrostatic latent image and the toner particles are sufficient to overcome the forces of attraction between the toner particles and the carrier particles. Thus, the electrostatic latent image is developed into a visible image.
It is possible to modify the triboelectric characteristics of the carrier particles by applying a coating material to the surface of the particles. It is then the coating on the particles rather than the particles themselves which determines the triboelectric characteristics. In the past, it has been possible to achieve the application to carrier particles of such coatings by tumbling the particles with the coating material or with a solution of the coating material and evaporation of the solvent from this solution.
Consideration has recently been given to the use of xerographic developers containing carrier particles of much smaller size than has until now been common. It has been found that the use of developers of this type can be particularly advantageous in high speed automatic xerographic machines. However, attempts to apply coatings to carrier particles of such small size, for example of the order of 250 microns, by conventional tumbling techniques have not been very successful because of the tendency of the particles to stick together during the tumbling operation. Since most carrier coating techniques are deficient in one or more of the above areas, there is a continuing need for an improved carrier coating process.